NASA is watching a dust storm on Mars the size of North America and Russia combined

This series of images from NASA's Opportunity rover on Mars shows the effects of a huge dust storm blotting out the sun on the Red Planet in June 2018. At left, the sun appears blindingly bright but darkens as the dust storm intensifies. At the far right, the sun is a mere pinprick, with the dust storm in full swing. Credit: NASA/JPL-Caltech/TAMU

NASA's Opportunity rover on Mars did not return a call from Earth Tuesday (June 12) while enduring a massive dust storm that scientists have called "one of the most intense ever observed."

"The Martian dust storm that has blotted out the sun above Opportunity has continued to intensify," NASA officials wrote in a mission update Tuesday. "The storm, which was first detected on May 30, now blankets 14 million square miles (35 million square kilometers) of Martian surface — a quarter of the planet." That means Opportunity's solar panels aren't getting enough sunlight to phone home from its Perseverance Valley location on Mars.

Opportunity's failure to reply to a signal from its mission operations center at NASA's Jet Propulsion Laboratory is a sign the rover has entered a "low power fault mode" in which all the rover's systems shut down except for a mission clock. That clock is programmed to wake up the rover periodically and check its power levels to see if it can call home. [The Greatest Mars Discoveries by Opportunity & Spirit]

"If the rover's computer determines that its batteries don't have enough charge, it will again put itself back to sleep," NASA officials said. "Due to an extreme amount of dust over Perseverance Valley, mission engineers believe it is unlikely the rover has enough sunlight to charge back up for at least the next several days."

The dust storm was first spotted on May 30 by NASA's Mars Reconnaissance Orbiter, and it has been growing ever since. NASA has continued to track the storm from orbit and the surface. In addition to Opportunity, the Mars rover Curiosity is monitoring dust levels from the storm.

Scientists think the power level in Opportunity's batteries is below 24 volts. If the rover loses power for an extended amount of time, there is a chance it won't be able to power the heaters needed to keep its electronics alive. The extreme cold of Mars is thought to be what killed Opportunity's twin, the Spirit rover, after it got stuck in Martian sand in 2010.

Opportunity has seen its share of dust storms on Mars.

In 2007, a planetwide dust storm blotted out the sun for two weeks. Opportunity went silent during that storm, too, but survived. The nearly 15-year-old rover launched in 2003 and has been exploring Mars since it landed in January 2004. Opportunity has lasted more than 50 times longer than its original 90-day mission plan, NASA officials have said.